In recent years, there are arguments concerning limitations of digital computers because of exponential increases in costs for equipment having increased speed. Analog computer are therefore drawing attention. On the other hand, stored conventional digital technology should be used and both workings of a digital data and on analog processings are necessary.
Recently, two types of circuits individually existed in the field of filter circuits: a circuit for treating analog data and a circuit for treating digital data.
In an analog circuit, there are three advantages: high speed processing time, small size by the fixed circuit and low price. On the other hand, there are advantages in a digital circuit, resulting from their simple, modifiable design and their inherent accuracy. There are also two disadvantages to digital circuits: large size of the circuit and high price by increasing investments for equipment.
No filter circuit currently in existence exhibits the advantages of both the analog and digital circuits while applying stored conventional digital technology.
Conventionally, to realize an adaptive filter, a programmable digital element such as DSP, etc. has been used. However, DSP is expensive and large size, so that a calculated result is fed back and multiple calculations are executed for a DSP. As a result, a filter with many taps cannot realize very high speed calculation. Also, a CPU was needed to control DSPs rendering it difficult to reduce the size of the total circuit.